Insertion Loss

By definition,¹ the "insertion loss at a given frequency caused by the insertion of apparatus in a transmission system is the ratio, expressed in decibels, of the powers at that frequency delivered to that part of the system beyond the point of insertion before and after the insertion."

A generalized equation that will give the insertion loss of any line, circuit, or device becomes quite involved and will not be given.^4,5 As an example, consider the insertion loss when a network, such as a few miles of telephone cable having a transmission loss (or attenuation loss, Chapter 7) within itself of L_N decibels, is inserted between a generator and a load. For these conditions, four losses will be involved in determining the insertion loss. These are the reflection loss L₁ at the junction of the network and the generator; the reflection loss L₂ at the junction of the network and the load impedance; the loss L_N within the cable (or network); and the reflection loss L₃ which would occur if the load impedance were connected directly to the generator. For this example the insertion loss is L₁ + L₂ + L_N - L₃, the loss L₃ being subtracted because it enters into determining the received power both before and after the insertion of the cable or other network.